1. Field of the Invention
The present invention relates to metal insulator semiconductor field effect transistors and, more particularly, to an improvement of a conductivity-modulation MOSFET.
2. Description of the related art
A conductivity-modulation MOSFET has a semiconductive drain layer, an N type conductivity base layer (N base region), and a base layer (P base region) of P type conductivity. The P base layer is formed by diffusion in an N type semiconductive layer serving as the N base layer. A heavily-doped N type layer (N.sup.+ layer) is formed on the P base layer to define a channel region of the MOSFET. A gate electrode layer insulatively covers the N base layer and the channel region, and a source electrode layer electrically shorts the P base layer and the N.sup.+ source layer. When a positive voltage is applied to the gate electrode, the channel region is inverted, so that carriers (electrons) are injected from the N.sup.+ source layer into the N base layer. When the electrons enter the drain layer through the N base layer, the PN junction of the MOSFET is forward-biased, and as a result, the MOSFET is turned on. When a zero or negative voltage is applied to the gate electrode, the inverted layer in the channel region disappears, and hence, the channel disappears. As a result the MOSFET is turned off.
In order to improve a turn-off switching speed of such a conductivity-modulation MOSFET, carriers accumulated in the N base layer thereof must disappear faster. However, with a conventional conductivity-modulation MOSFET, if the carrier lifetime in the N base layer is shortened in order to improve the turn-off speed, a voltage in the device becomes undesirably high, which leads to the difficulty in turn-on drive of the MOSFET.